The present invention relates generally to the use of laser radiation as a therapeutic tool in medicine and surgery, and more particularly to the use of an infrared laser in the precision surgical ablation or cutting of tissue under conditions where minimization of damage to adjacent non-targeted tissues is required.
Laser technology is currently used in clinical medical practice in a variety of applications, including as a surgical tool for the therapeutic ablation of human tissues, both internal and external. In some applications, the precision obtainable by a narrowly and accurately focused beam of laser radiation is superior to other more traditional surgical techniques. However, the use of lasers in certain areas, such as in the eye or brain, carries also the risk of thermal damage being done to sensitive tissues adjacent to the areas where tissue incision or removal is desired.
Although prior art laser surgery techniques have recognized the problems of thermal damage to healthy tissues during laser surgery, none of the proposed solutions have been entirely satisfactory. A principal deficiency is that prior art laser surgery techniques have not employed the optimum non-photochemical wavelengths of laser radiation which produce ablation without thermal damage. The infrared (IR) region has been preferred over ultraviolet (UV) in many surgical applications because the IR wavelengths are non-photochemical in their effect on tissue and because laser radiation at some UV wavelengths has been reported to cause cell mutation.
For example, CO.sub.2 lasers are in common surgical use and have a nominal operating wavelength of 10.6 microns. Unfortunately, experience has demonstrated that thermal damage to healthy adjacent tissue is a predictable consequence of the use of a CO.sub.2 laser to ablate tissue. Although selection of an appropriate pulse structure and duty cycle can improve the effectiveness of CO.sub.2 lasers in some applications, the damaged areas cannot be eliminated.
Other investigators and practitioners have used Er:YAG or other solid state lasers in the infrared range, often tuned to a wave length which is known to correspond to an energy absorption band of water, at 2.94 microns for example. The theory behind methods which use such laser wavelengths is that because human tissue is approximately eighty percent (80%) water, the interaction of laser energy in water will also characterize the response of human tissues to infrared radiation of the same wavelength. The O--H stretch mode of water, which corresponds to 2.94 microns, is the most efficient absorber of IR radiation. This theory (and these prior art methods), however, fail to take into account and properly compensate for the method of energy transfer into a biomaterial, in this case human or animal tissue, which universally includes one or more proteins and related structures which both confine and are affected by water vaporization.
Thus, the use of a laser surgery method which maximizes transfer of energy into the water component of human tissue by targeting the O--H stretch or other vibrational modes of water may produce rapid and effective ablation of tissue. The problem of heating of water and consequent thermal and dynamic effects on the structure of adjacent tissues remains. Accordingly, those experienced in the art have reported that ablation of tissue using a laser tuned to 2.94 microns is achieved by an explosive mechanism involving rapid heating, vaporization, and subsequent high-pressure expansion of irradiated tissue. It is believed, then, that this mechanism can cause thermal damage to collateral tissues from the hot gases produced, and tearing of those same tissues by pressures exerted by both the expanding gases and liquified or vaporized tissue from the target area. Similar thermal and mechanical effects have been reported at shorter wavelengths and at 10.6 microns. This ablation mechanism is particularly undesirable in situations, such as in delicate eye surgery, where precision liquification and extraction of tissue is preferred over explosive vaporization.
What is needed, then, is a laser surgery method which can efficiently and precisely ablate a variety of human tissue types with controllable, and is some cases, little or no discernable thermal or mechanical damage to non-targeted adjacent tissues. Such a method is presently lacking in the prior art.